I have two drives in my Win7 PC. C: (OS, programs) is 500GB, and D: (some installed software, Steam/GOG libraries, mostly data) is 2TB but I'm barely using 1/10th of it.

I'd like to migrate the data from the 2TB drive onto a smaller one, and then repurpose it as a backup drive. So I'd have:

C: - 500GB (OS/boot)
D: - 500GB or ~1TB
Extra 2TB drive for backups

So what I need is something equivalent to Apple's Time Machine (or Linux's Timeshift), that can mirror both C & D onto the third drive and track changes/deletions/etc. AFAIK Windows doesn't have anything like this built in?

I don't necessarily need seamless 1-click bare-metal restore or anything, I just want a reliable mirror of the files on the disks.

What's a good 3rd party app to do this? What do you guys use?

(Note: before anyone suggests it, I have a maximum of 3 drive bays in the system, so I really do need to mirror the two main drives onto a single third one, which rules out adding two more drives as RAID1 mirrors. Also, I don't want to start changing around the C/D configuration I already have, i.e. by copying all the files onto one drive or re-partitioning. It's taken me a LONG time to get everything installed & set up the way I like.)